1. Field of the Invention
The present invention relates to an apparatus for forming a mixture of reducing agent and exhaust gas and to an exhaust gas cleaning system using the apparatus.
2. Description of the Prior Art
Exhaust gas cleaning systems, particularly for reducing nitrogen oxides (NOx) in the exhaust gas of Diesel engines, are known. One such exhaust gas cleaning system is described in German Patent DE 197 31 865 C2, for instance. A catalytic converter assembly used here has an oxidation catalytic converter, in which the NO present in the exhaust gas is first converted into NO2. In a downstream reservoir component, the nitrogen oxides are then stored, for instance in the form of barium nitrate. In order to maintain the reservoir component so that it is functional over a long period, it is advantageous to regenerate it from time to time. For this regeneration, it is necessary to achieve so-called rich phases in the exhaust gas stream, that is, to subject the exhaust gas stream to a suitable reducing agent. The enrichment can be done for instance by mixing in fuel (HC). The rich phases can be accomplished within the engine in the lower rpm and load range. At higher rpm and torque levels, this can be achieved, without drawbacks, only with an external metering in of reducing agent, for instance. Postinjecting reducing agent at high injection pressures in the combustion chamber has the drawback of diluting the oil. Metering directly into the exhaust gas system through an injection nozzle and preheating the fuel, for instance with a spark plug, has so far not achieved satisfactory results, because the fuel temperature is too low and its preparation is inadequate.
It has proved to be disadvantageous that if reducing agents are overdosed, in an SCR catalytic converter, unwanted exhaust gas components occur. The occurrence of such byproducts is due among other factors also to incomplete aerosol formation of the reducing agent in the exhaust gas. It has been attempted in this respect to position mixers, such as so-called Sulzer mixers, in the exhaust gas system to improve the aerosol formation. However, since the volumetric flow of exhaust gas in the Diesel engine is highly dependent on the engine rpm and engine load, such mixers must be provided with controllable blades or swirl generators, and it is found that such controls are relatively complicated to produce and to maintain. On the other hand, with noncontrollable mixers, when flow velocities are high, demixing of reducing agent and exhaust gas can occur because too much swirl is created, so that the reducing agent is deposited onto the wall of the exhaust pipe.